The disclosure relates to a method for semiconductor processing. More specifically, the disclosure relates to a method for improving selectivity in a selective deposition process.
Fabrication of semiconductor devices typically requires interconnect structures that include metal wiring to connect devices in a semiconductor chip. The wiring includes metal lines formed in dielectric layers that extend across the chip as well as vias that connect the lines on different levels of the chip. In many applications, selective deposition is performed on substrates to form vias. The metal lines and vias are typically formed of aluminum or copper and are insulated by the dielectric.
In semiconductor fabrication, it is desirable to have a via that is fully aligned to both levels to which the via connects. When drilling down to form vias to metal lines below, if there is any misalignment, the via could potentially miss the desired metal line and instead go to another line, thereby causing a short.
In a process to form fully aligned vias, aluminum oxide (Al2O3) is typically selectively deposited on the dielectric surface as an etch stop layer after the metal lines (i.e., copper) are deposited. It is known that Al2O3 will only deposit on a dielectric surface (e.g., silicon oxide) and not on a pristine copper surface that has been exposed to inhibitor molecules. Forming vias using this selective deposition process, however, requires a pristine copper surface, such as is deposited using physical vapor deposition (PVD), which requires high operating temperatures and is also difficult for depositing on complex geometries. Current technology for modifying a copper surface to improve selectivity employs a two-step process: (1) exposing the pristine copper surface to an inhibitor; and (2) depositing Al2O3 using thermal atomic layer deposition (ALD). This process only works with copper deposited by PVD and not with copper deposited by either electrofill (EF) or EF followed by chemical mechanical polishing (CMP).
However, in processes for forming fully aligned vias, the copper is typically deposited using an EF process and is then chemically mechanically polished to form the vias. This copper deposited by the EF/CMP process does not adsorb inhibitor molecules and a subsequent ALD Al2O3 process to deposit an etch stop layer would therefore not be selective. Therefore, it would be desirable to improve selectivity in a deposition process on copper deposited by EF/CMP in a process for forming fully aligned vias.